Made generic Entity that always has a hitbox.
Make HalfExtents and CenterOffset automatically sent over by the server.
Give the player entity info as part of the handshake.

This will allow servers to spawn the player as an arbitrary entity
(at least, once we support multiple entities), and is a step toward
removing duplication of some player geometry constants.
Fix client prediction.
Remove PlayerSize and PlayerEntity.

PlayerSize is now communicated by the server, it's no longer (necesarilly)
constant. PlayerEntity was basically just a wrapper around Entity that
knew the size of a player; now that PlayerSize is gone there is no need
for PlayerEntity.
Match go naming conventions.
Make the player fit within their bounding box.

Used an atomical model, so it should theoretically be accurate,
but it actually ends up looking a bit more like slenderman or
something. We might want to tweak it a bit in the future, or
maybe adding legs will help?
Don't use cryptic acronyms.
Cleanup logs a bit.
Gofmt.
Merge pull request #140 from crazy2be/bugfix-fix-hitbox-size-127

Looks good, takes 30 pounds off my figure.